The objective of the proposed research is to further understanding of the mechanisms by which murine B lymphocytes differentiate into immunocompetent cells during development, and to analyze some of the regulatory mechanisms involved in B cell triggering, in particular regulation mediated by genes coded by the I-region of the murine major histocompatibility complex. Genetically determined and serologically defined cell surface molecules will be used as phenotypic markers to follow B cell differentiation during embryonic, neonatal, and adult life; and to examine the acquisition and expression of antibody specificity during B cell development, including responses to antigens which are under immune response (Ir) gene control. The potential role of lymphocyte cell surface glycoproteins in controlling antigen-specific B cell stimulation will be examined.